In a slurry pipeline it is often necessary to provide back pressure to avoid damage to the pipeline. This back pressure is usually provided by one or more orifice chokes located to limit the volume rate of flow of the slurry at the outlet of the pipeline. If the back pressure must be varied, the system requires more than one orifice choke.
In a conventional slurry pipeline system, the orifice chokes are placed in bypass pipes connected to a main or central pipe with the bypass pipes being in a horizontal plane. By controlling a number of valves near the bypass pipes, the slurry flow can be directed through one or more of the bypass pipes to vary the back pressure. The bypass pipes are used because, under certain hydraulic circumstances, the main pipe pressure upstream of the bypass pipes falls below the vapor pressure of water which causes the fluid to cavitate and flow at a higher velocity while only partially filling the main pipe. This high velocity and cavitation erodes the pipe at an undesirable rate. To prevent this, the slurry flow is shunted into one or more of the bypass pipes, each of which contains a choke to increase the back pressure in the pipe.
Because the bypass pipes are in horizontal planes, they cannot adequately drain so that solids are collected in them and totally or partially plug the bypass pipes. This requires the system to be cleaned out frequently to assure continuous operation. Moreover, the arrangement of bypass pipes in horizontal planes requires a relatively large number of valves; specifically, at least three valves for each bypass pipe, respectively. For achieving a wide range of back pressures, a relatively large number of control valves are necessary, thereby increasing the cost and complexity of the conventional back pressure systems. A need has, therefore, arisen for an improved system for providing back pressure in a slurry flow in a simple manner while avoiding the problem of plugging as occurs in conventional systems.